Flow meters are commonly used devices for measuring a flow of a medium through a pipe. In various branches of industry, for example in the chemical industry or in the pharmaceutical industry, measured flow indications obtained by flow meters are used to control complex industrial processes.
There is a wide variety of flow meters on the market, operating on various different measurement principles, like for example ultrasonic flow meters, coriolis flow meters, vortex flow meters or electromagnetic flow meters.
In order to ensure, that flow meters fulfill a specified measurement accuracy, they are calibrated before they are first installed. Calibration is a process during which measurement indications obtained with the flow meter to be calibrated under predefined conditions are compared to a corresponding reference or standard. To this extend the accuracy with which the corresponding reference or standard needs to be known or determined has to be at least as accurate, preferably more accurate, than the measurement accuracy specified for the flow meter under test. Flow meter calibrations are thus regularly performed on specially designed calibration rigs, capable of producing an accurately determinable flow through the flow meter under test and/or capable of sending an accurately determinable quantity of a medium through the flow meter under test.
In case calibration shows that the measured flow indications of the flow meter under test are less accurate than specified, the flow meter is adjusted based on the calibration data, and re-calibration is performed, until the achieved measurement accuracy complies to the specified measurement accuracy. Compliance is for example determined by comparing the largest measurement indication error, determined during calibration, to a maximal permissible error. The maximal permissible error is the largest measurement indication error, that a flow meter may have, whilst still in compliance the specified measurement accuracy.
The flow meter is then installed on a measurement site and taken into operation. During the life time of a flow meter its measurement properties may change due to aging, environmental conditions, or other effects.
Thus in order to ensure the specified measurement accuracy during long term operation of the flow meter, flow meters are re-calibrated regularly, e.g. once a year. Since re-calibration generally requires the use of specially designed calibration rigs, re-calibration cannot be performed in line during normal operation of the flow meter at the measurement site. Thus re-calibration is time and cost intensive, since it usually requires an interruption of the ongoing production process at the measurement site.
To avoid or at least reduce the time and cost involved in re-calibration, it is feasible to install an additional flow meter in line with the flow meter under consideration, and to verify proper operation of the flow meter based on a deviation between the measured flow indications of the two flow meters on site. In this respect non-invasive ultrasonic flow meters, comprising ultrasonic transducers to be clamped on the outside of the pipe, would be most suitable since they can be easily mounted on site on demand without any interruption of the ongoing production process on the site, and properly re-calibrated before every verification.
Under favorable conditions ultrasonic flow meters achieve a measurement accuracy of about 2% of the measured flow. Thus they render much less accurate measurement results than for example Vortex flow meters typically achieving a measurement accuracy in the range of 1%, electro-magnetic flow meters typically achieving a measurement accuracy in the range of 0.2%-0.5%, or Coriolis flow meters achieving a measurement accuracy in the range of 0.1%. The deviation between the measured flow indication QUS of the ultrasonic flow meter and the measured flow indication QFL of the flow meter to be verified is determined as their difference Δ given by:Δ=QFL−QUS  (1)
Thus the standard measurement uncertainty of this deviation Δ is assessed according to the law of propagation of uncertainty described e.g. in the ISO Guide 98-3 of the international organization of standardization to be equal to:uΔ=√{square root over (uFL2+uUS2)}  (2)
wherein
uFL is a standard measurement uncertainty of the flow meter, and
uUS is a standard measurement uncertainty of the ultrasonic flow meter.
Thus for an ultrasonic flow meter with a measurement accuracy of 2% and a flow meter with a measurement accuracy of 0.5% the standard uncertainty of the deviation will amount to 1.03%, when considering a standard measurement uncertainty equal to half the measurement accuracy. Thus an expanded uncertainty corresponding to this assumption amounts to 2.03%. Therefore only deviations Δ between corresponding measurement indications of the two flow meters of more than 2.06% of the measured flow can be identified as being due to impaired measurement properties of the flow meter to be verified, with an acceptable level of confidence. Smaller deviations do not render information on the measurement properties of the flow meter to be verified, since they are within the measurement uncertainty of the verification method.